Engines may be configured with an exhaust heat recovery system for recovering heat of exhaust gas generated at an internal combustion engine. The heat is transferred from the hot exhaust gas to a coolant through an exhaust gas heat exchanger system. The heat from the coolant, circulated through the an exhaust gas heat exchanger, may be utilized for functions such as heating the cylinder head, and warming the passenger cabin, thereby improving engine efficiency. In hybrid electric vehicles, the recovery of exhaust heat improves fuel economy by enabling engine temperatures to be maintained longer, thereby allowing for a faster engine shut-off and extended use of the vehicle in an electric mode.
Exhaust heat may also be retrieved at an exhaust gas recirculation (EGR) cooler. An EGR cooler may be coupled to an EGR delivery system to bring down the temperature of recirculated exhaust gas before it is delivered to the intake manifold. EGR may be used to reduce exhaust NOx emissions. Further, EGR may be used to assist in the reduction of throttling losses at low loads, and to improve knock tolerance.
Various approaches are provided for exhaust heat recovery and EGR cooling. In one example, as shown in US 20120260897, Hayman et al. discloses an engine system wherein exhaust gas from a first group of cylinders is directed to an exhaust tailpipe. Exhaust gas from a second group of cylinders is directed to an intake manifold via an EGR passage. In addition, based on the EGR requirement of the engine, a portion of the exhaust from the second group of cylinders may be diverted to the exhaust tailpipe via a bypass passage. An EGR cooler coupled to the EGR passage is used to cool the exhaust gas prior to mixing of the EGR with combustion air and entry of the mixture into the intake manifold.
However, the inventors herein have recognized potential disadvantages with the above approaches. As one example, it may be difficult to coordinate EGR cooling with exhaust heat recovery. In particular, the heat recovered at the EGR cooler cannot be used for heating a cabin space. As a result, a distinct heat exchanger is required for cabin heating. Likewise, even if heat is extracted from exhaust gas at a heat exchanger, the cooled exhaust gas is not recirculated, resulting in the need for a distinct EGR cooler. The additional components add cost and complexity.
The inventors herein have identified an approach by which the issues described above may be at least partly addressed. One example method for an engine comprises: operating in a first mode with exhaust gas flowing through an exhaust bypass, and in a first direction through a heat exchanger coupled in the exhaust bypass, and then to an exhaust tailpipe; and operating in a second mode with exhaust flowing through an exhaust passage, then in a second, opposite direction through the heat exchanger, and then to the engine intake. In this way, EGR cooling and exhaust heat recovery can be provided via a common heat exchanger.
In one example, an engine system may be configured with a heat exchanger positioned downstream of a catalytic convertor in an exhaust bypass disposed parallel to a main exhaust passage. A pair of butterfly valves may be used to enable exhaust gas to be diverted into the bypass passage, and through the heat exchanger in one of two directions, a position of the valves adjusted based on engine operating parameters. For example, during conditions when EGR is not required, the valves may be adjusted to flow exhaust through the bypass passage in a first direction through the heat exchanger, and then on to the exhaust tailpipe. During the flow, exhaust heat is transferred to the heat exchanger, wherefrom the heat is transferred to a coolant circulating around the heat exchanger, the hot coolant then used for functions such as cabin heating. In comparison, when cooled EGR is required, the valves may be adjusted to flow exhaust gas through the bypass passage and across the heat exchanger in a second direction opposite to the first direction before the exhaust gas is recirculated to the engine intake via an EGR passage. The heat recovered at the heat exchanger during this flow is transferred to circulating coolant, and thereon the warmed coolant may be circulated back to the engine (such as when engine heating is required) and/or utilized for heating a passenger cabin of the vehicle (such as when cabin heating is requested). Alternatively, the extracted heat is transferred to the radiator for dissipation into the atmosphere. Further, during conditions when hot EGR is required, the valves may be adjusted so that a portion of the exhaust gas can be diverted to the bypass passage, and drawn into the EGR passage before passing through the heat exchanger.
In this way, the heating requirements of an engine system may be met using a single heat exchanger. By providing the functions of an EGR cooler and an exhaust gas heat exchanger via a single heat exchanger, cost and component reduction benefits are achieved without limiting the functionality or capability of either system. In addition, the specific configuration of a single heat exchanger in a bypass exhaust passage allows for a shorter EGR passage length, which reduces EGR transport delays. The technical effect of using a pair of butterfly valves to regulate the passage of exhaust gas through the bypass passage is that exhaust gas can be flowed in both directions across the heat exchanger. As such, this improves the heat transfer efficiency and reduces the requirement for long EGR passages. Overall, by improving the amount of waste heat that can be recovered from exhaust using fewer components, engine fuel economy and performance is improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.